Solar cells utilizing sunlight are watched as energy sources to replace fossil fuels, and various studies have been made. The solar cells are a sort of photoelectric converters for converting light energy into electric energy and use sunlight as an energy source. Therefore, the solar cells are extremely small in influences against the global environment, and much more diffusion thereof is being expected.
As to the principle and materials of solar cells, various investigations have been made. Above all, solar cells utilizing pn junction of a semiconductor are most diffused at present, and a number of solar cells containing silicon as a semiconductor material are commercially available. These are roughly classified into a crystalline silicon based solar cell using monocrystalline or polycrystalline silicon and an amorphous silicon based solar cell using amorphous silicon.
As to the photoelectric conversion efficiency expressing the performance for converting light energy of sunlight into electric energy, the crystalline silicon based solar cell is higher than the amorphous silicon based solar cell, and therefore, the crystalline silicon based solar cell has hitherto been frequently used for the solar cell. But, since the crystalline silicon based solar cell requires a lot of energy and time for the crystal growth, the productivity is low, leading to high costs.
On the other hand, the amorphous silicon based solar cell has characteristics such that it is able to absorb and use light in a wider wavelength region as compared with the crystalline silicon based solar cell; and it is able to choose a substrate material of every kind so that it is easy to realize a large area. Also, the amorphous silicon based solar cell does not require crystallization so that it can be manufactured with good productivity and at low costs as compared with the crystalline silicon based solar cell. But, its photoelectric conversion efficiency is lower than that of the crystalline silicon based solar cell.
In all of these silicon based solar cells, since a step of manufacturing a high-purity semiconductor material and a step of forming pn junction are needed, there is involved a problem that the number of manufacturing steps becomes large; and since a manufacturing step under vacuum is needed, there is involved a problem that the equipment costs and energy costs are high.
In order to realize a solar cell which is free from the foregoing problems and which can be manufactured at lower costs, solar cells using an organic material in place of the silicon based material have been long studied. But, in the majority of them, the photoelectric conversion efficiency is low as about 1% so that they have not been put to practice use yet.
However, a dye sensitization photochemical battery (photoelectric converter) applying photoinduced electron transfer was proposed in 1991 (see, B. O'Regan, M. Graetzel, Nature, 353, pages 737 to 740 (1991) and Japanese Patent No. 2664194 (pages 2 and 3 and FIG. 1), etc.). Since this photoelectric converter has high photoelectric conversion efficiency, does not require a massive manufacturing system and can be simply manufactured with good productivity using cheap materials, it is expected as a next-generation solar cell.
FIG. 10 is a cross-sectional view of the principal part illustrating a structure of an existing general dye sensitization photoelectric converter 100. The dye sensitization photoelectric converter 100 is mainly configured of a transparent substrate 1 such as glass, a transparent electrode (negative electrode) 2 composed of a transparent conductive layer such as FTO (fluorine-doped tin(IV) oxide SnO2), a semiconductor layer 103 holding a single kind of a photosensitizing dye, an electrolyte layer 5, a counter electrode (positive electrode) 6, a counter substrate 7 and a (non-illustrated) sealing medium, etc.
As the semiconductor layer 103, a porous layer obtained by sintering fine particles of titanium oxide TiO2 is frequently used. A single kind of a photosensitizing dye is held on the surface of the fine particles configuring this semiconductor layer 103. The electrolyte layer 5 is filled between the semiconductor layer 103 and the counter electrode 6, and an organic electrolytic liquid containing an oxidation-reduction species (redox pair) such as I−/I3− is used. The counter electrode 6 is configured of a platinum layer 6b, etc. and formed on the counter substrate 7.
FIG. 11 is an energy diagram for explaining the actuation principle of the dye sensitization photoelectric converter 100. When light is made incident, the dye sensitization photoelectric converter 100 is actuated as a cell in which the counter electrode 6 acts as a positive electrode, and the transparent electrode 2 acts as a negative electrode. Its principle is as follows. (In FIG. 11, it is supposed that FTO is used as a material of the transparent electrode 2; N719 as described later is used as a photosensitizing dye 104; titanium oxide TiO2 is used as a material of the semiconductor layer 103; and an oxidation-reduction species of I−/I3− is used as the redox pair.)
When the photosensitizing dye 104 absorbs a photon which has transmitted through the transparent substrate 1 and the transparent electrode 2, an electron in the photosensitizing dye 104 is excited from a ground state (HOMO) to an excited state (LUMO). The electron in an excited state is drawn out into a conduction band of the semiconductor layer 103 via electrical coupling between the photosensitizing dye 104 and the semiconductor layer 103 and passes through the semiconductor layer 103 to reach the transparent electrode 2.
On the other hand, the photosensitizing dye 104 which has lost an electron receives an electron from a reducing agent in the electrolyte layer 5, for example I−, according to the following reaction:2I−→I2+2e−I2+I−→I3−forms an oxidizing agent, for example, I3− (coupled body of I2 and I−) in the electrolyte layer 5. The formed oxidizing agent reaches the counter electrode 6 due to diffusion and receives an electron from the counter electrode 6 according to the following reaction which is a reverse reaction to the foregoing reaction:I3−→I2+I−I2+2e−→2I−and is reduced to the original reducing agent.
The electron which has been sent out from the transparent electrode 2 to an external circuit electrically works in the external circuit and then returns to the counter electrode 6. In this way, the light energy is converted into the electric energy without leaving any change in both the photosensitizing dye 104 and the electrolyte layer 5.
As the photosensitizing dye 104 of the dye sensitization photoelectric converter 100, substances capable of absorbing light in the vicinity of a visible light region, for example, bipyridine complexes, terpyridine complexes, merocyanine dyes, porphyrin, phthalocyanine, etc. are usually used.
In general, in order to realize high photoelectric conversion efficiency, it has hitherto been considered to be favorable to use a single kind of a dye with high purity. This is because it is considered that in the case where plural kinds of dyes are intermingled on the single semiconductor layer 103, transfer of an electron between the dyes each other or recoupling between an electron and a hole occurs, or an electron which has been transferred from an excited dye to the semiconductor layer 103 is captured by a dye of a different kind, whereby an electron reaching the transparent electrode 2 from the excited photosensitizing dye 104 decreases, and a ratio for obtaining current from the absorbed photon, namely a quantum yield remarkably lowers (see, for example, K. Hara, K. Miyamoto, Y. Abe, M. Yanagida, Journal of Physical Chemistry B, 109(50), pages 23776 to 23778 (2005), “Electron Transport in Coumarin-Dye-Sensitized Nanocrystalline TiO2 Electrodes”; Masatoshi Yanagida, et al., Symposium on Photochemistry (2005), 2P132, “Electron Transport Process in Dye-Sensitized Nanocrystalline Titanium Oxide Electrodes Having Ruthenium Bipyridine Comlex and Ruthenium Biquinoline Complex Coadsorbed Thereon”; Uchida, http://kuroppe.tagen.tohoku.ac.jp/˜dsc/cell.html, FAQ, “Re: Theoretical Efficiency of Dye-Sensitized Solar Cells”; etc.).
As the dye to be used singly, a cis-bis(isothiocyanate)bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) ditetrabutylammonium complex (commonly called as “N719”) which is a sort of bipyridine complexes is excellent in performance as the sensitizing dye and is generally used. Besides, cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) (commonly called as “N3”) which is a sort of bipyridine complexes and a tris(isothiocyanato)(2,2′:6′,2″-terpyridyl-4,4′,4″-tricarboxylate)ruthenium(II) tritetrabutylammonium complex (commonly called as “Black Dye”) which is a sort of terpyridine complexes are generally used.
In particular, when N3 or Black Dye is used, a coadsorbent is frequently used. The coadsorbent is a molecule which is added for the purpose of preventing association of a dye molecule on the semiconductor layer 103 from occurring, and representative examples of the coadsorbent include chenodeoxycholic acid, taurodeoxycholic acid salts and 1-decrylphosphonic acid. Examples of structural characteristics of these molecules include the matter that they have a carboxyl group, a phosphono group, etc. as a functional group which is easily adsorbed on titanium oxide constituting the semiconductor layer 103; and the matter that in order that they may lie between dye molecules to prevent interference between the dye molecules, they are formed by a σ-bond.
In general, in order to effectively actuate a photoelectric converter, first, it is important to enhance the light absorptance such that light coming into the photoelectric converter can be utilized at a maximum; and next, it is important to enhance the conversion efficiency for converting absorbed light energy into electric energy. In a dye sensitization photoelectric converter, since the light absorption is borne by the photosensitizing dye 104, it is expected that the maximum light absorptance can be realized by choosing, as the photosensitizing dye 104, a dye having optimal light absorbing properties against the incident light.
Since sunlight includes lights with various wavelengths continuously from infrared light to ultraviolet light, when applied as a solar cell, in order to realize a high light absorptance, it is desirable to choose a dye capable of absorbing exhaustively light of a wavelength region over the range as wide as possible including a long-wavelength region, especially light having a wavelength of from 300 to 900 nm.
However, the state of the electron in the photosensitizing dye 104 is quantum mechanically determined, and energy states other than that inherent to its substance cannot be taken. In consequence, a difference in energy between the electron in a ground state (HOMO) and the electron in an excited state (LUMO), namely energy (band gap energy) necessary for exciting the electron from the ground state to the excited state, is also determined as a value inherent to its substance, and the light which the photosensitizing dye 104 is able to absorb corresponding thereto is restricted to light in a specified wavelength region. Also, in order that the excited electron may be able to move into a conduction band of the semiconductor layer 103, it is required that the band gap energy of the dye does not become too small.
FIG. 12(a) is a graph showing absorption spectra of four kinds of representative dyes which are generally available at present; and FIG. 12(b) is a graph showing enlarged absorption spectra of three kinds of dyes having a small molar absorption coefficient. It is noted from FIG. 12 that Black Dye has a wide-ranging absorption wavelength region including a long-wavelength end in the vicinity of 860 nm. But, its molar absorption coefficient is small as a whole, and in particular, a region where the absorbance is insufficient is present on the short-wavelength side. Though N719 has a molar absorption coefficient equal to or more than Black Dye on the short-wavelength side, its long-wavelength side end of the absorption wavelength region is present in the vicinity of 730 nm, and light having a long wavelength cannot be effectively utilized. The light absorption by 5-[[4-[4-(2,2-diphenylethenyl)phenyl]-1,2,3,3a,4,8b-hexahydrocyclopent[b]indol-7-yl]methylene]-2-(3-ethyl-4-oxo-2-thioxo-5-thiazolidinylidene)-4-oxo-3-thiazolidineacetic acid (hereinafter referred to as “Dye B”) has wavelength dependency substantially the same as N719, and its molar absorption coefficient is smaller than that of N719. Though 2-cyano-3-[4-[4-(2,2-diphenylethenyl)phenyl]-1,2,3,3a,4,8b-hexahydrocyclopent[b]indol-7-yl]-2-propenoic acid (hereinafter referred to as “Dye A”) has a large molar absorption coefficient, its absorption wavelength region is restricted narrow.
As described previously, a dye which is able to exhaustively absorb sunlight having a wavelength of from 300 to 900 nm does not exist at present. In the case where the dye sensitization photoelectric converter 100 is used as a solar cell, the maximum performance is achieved in the case of using N719 as the photosensitizing dye 104, and for example, a performance of 0.755 V in open-circuit voltage and 8.23% in photoelectric conversion efficiency is obtained. When this result is compared with a performance of 0.6 V in open-circuit voltage and 15% in photoelectric conversion efficiency as achieved in a crystalline silicon based solar cell, the photoelectric conversion efficiency is no more than a little over a half.
Taking into consideration the matter that the dye sensitization photoelectric converter 100 is larger in the open-circuit voltage than the crystalline silicon based solar cell, it is considered that the reason why the photoelectric conversion efficiency of the dye sensitization solar cell is low resides in the matter that the obtained photocurrent is remarkably small as compared with that in the crystalline silicon based solar cell; and that its primary cause resides in the matter that the light absorptance by the photosensitizing dye 104 is insufficient. That is, it is considered that since a dye which is able to efficiently absorb all lights with various wavelengths to be included in sunlight does not exist, the light absorptance is insufficient in the dye sensitization solar cell composed of a single kind of a dye.
If sufficient light absorption cannot be realized in a single kind of a dye, it may be considered to use plural kinds of dyes having different absorption wavelength properties from each other as a photosensitizing dye. However, as described previously, when plural kinds of dyes are intermingled on the single semiconductor layer 103 and used, the photoelectric conversion efficiency is actually lowered in almost all of cases. This is because as described previously, a ratio for obtaining current from the absorbed photon by the electron transfer between the dyes or the like, namely a quantum yield remarkably lowers.
In view of the foregoing circumstances, the present invention has been made, and its object is to provide a dye sensitization photoelectric converter with enhanced light absorptance and photoelectric conversion efficiency.